Solved Numerical Problems | Chapter 6 | Work & Energy | 9th Physics | National Book Foundation

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The unit of work or energy joule (j) is equal to
(a) horsepower (b) watt metre (c) watt second (d) newton second
A car, an elephant and a cricket ball have same kinetic energies. Which of these will have a greater speed?
(a) car (b) elephant (c) cricket ball (d) all have same speed
A ball weighing 50N is lifted to a height of 5 metre. The potential energy stored in it is:
(a) 10J (b) 25J (c) 45J (d) 55J
What is the power utilized when 100J of work is done in 5s.
(a) 10W (b) 20W (c) 105W (d) 500W
The SI unit of power is
(a) joule (b) watt (c) horsepower (d) erg

A car is moving with a constant speed along a straight road. Is there any work done on the car?
Does the work done in raising a box up in a building depend upon how fast it is raised up? Through which path? To how much height?
Work done on the body either speeds it up, slows it down. Keeping it mind, explain how much work is done by centripetal force on an orbiting satellite?
A car has kinetic energy E_K. By what factor its kinetic energy would change, if its velocity is doubled?
A bullet is fired from gun, bullet penetrates into sand wall and it stops. Where does its kinetic energy used?
An LED light bulb has efficiency of 80%. Does it violate conservation of energy principle?
How does using renewable energy sources contribute to reducing environmental impact compared to non-renewable sources?
Will we eventually rely entirely on renewable energy sources? Why or why not?
How can increasing the power of a machine impact its energy consumption?
A perpetual engine has an efficiency equal to 1. Why it will not work?
Calculate the work done in pushing a box with 150N through distance of 5m.
A boy weighing 75N jumps up and gains 300J of gravitational potential energy. Find the height to which the boy will rise.
A 5kg steel ball is dropped from the top of a 12m high tower. What is kinetic energy of the ball on hitting the ground? With what velocity will it hit the ground? (Neglect resistance).
A 2.0kg rock is dropped from 20m tall building. What is the kinetic and gravitational potential energy when the rock has fallen 15m.
A rocket with a mass of 800g is launched vertically upward with an initial speed of 30m/s. (a) Assuming no air resistance, calculate the maximum height the rocket would reach. (b) If, due to air friction, the rocket only rises to 25m, determine the work done against air resistance.
A 2hp electric motor gives energy to a system that lifts a load of 100kg to height of 10m in 1.5s. Calculate (a) Input (Work done by motor on system) (b) Output (load lifted by system) and (c) Efficiency of the system.
What horsepower (hp) is required to pump up 2500kg of water to 100m height in 5 minutes?
6.1 Muhammad Tauha was pushing a box with a force of 100N on a flat frictionless surface. How much work he does on the box if he pushes it through a displacement of 200m?
6.2 Baber Azam hits a cover drive by giving kinetic energy of 50J to the ball by his bat. At what speed will the ball go to the boundary if mass of the ball is 120g?
6.3 At School sports gala, Kamil made a record in high jump at his school. He jumped 1.5m high. What will be his potential energy if his mass is 60kg? From where does this potential energy come?
6.4 A player kicks the football of mass 450g is thrown vertically upward at a speed of 𝟐𝟐〖𝒎𝒔〗^(−𝟏). (a) Neglecting air resistance how high would it reach. (b) If due to air friction the ball only rises to 20m, what is the work done against air resistance?
6.5 How powerful will be a Tesla Car’s engine if it does 𝟑×〖𝟏𝟎〗^𝟕 𝑱 of work in 1 minute? What will be its power in hp?